In mobile telecommunication systems of the time division multiple access (TDMA) type, time-division communication takes place on the radio path in successive TDMA frames, each of which consists of several time-slots. In each time-slot, a short information packet is sent in form of a radio frequency burst which has a finite duration and which consists of a set of modulated bits. The time-slots are mainly used for conveying control channels and traffic channels. On the traffic channels, speech and data are transmitted. On the control channels, signalling between a base station and mobile subscriber stations is carried out. An example of a TDMA radio system is the Pan-European mobile communication system GSM (Global System for Mobile Communications).
For communication in conventional TDMA systems, each mobile station is assigned one channel time-slot for data or speech transmission. Thus, the GSM system, for instance, may have up to eight parallel connections to different mobile stations on a same carrier frequency. The maximum data transfer rate on one traffic channel is restricted to a relatively low level, depending on the available channel bandwidth, as well as the channel coding and error correction used in the transmission. In the GSM system, for instance, the user rate is 9.6 kbit/s or 12 kbit/s. In addition, in the GSM system a so-called half-rate (max. 4.8 kbit/s) traffic channel may be chosen for low speech coding rates. A half-rate channel is established when a mobile station communicates in a certain time-slot only in every second time-slot, i.e. at half-rate. A second mobile station communicates in every second time-slot of the same frame. The capacity of the system in terms of the number of subscribers may thus be doubled, in other words, up to 16 mobile stations may communicate simultaneously on the same carrier frequency.
In the last few years, the need for high-speed data services in mobile communication networks has remarkably increased. Data transfer rates of at least 64 kbit/s would be required for utilizing ISDN (Integrated Services Digital Network) circuit switched digital data services, for example. Data services of the public network PSTN, such as modems and telefax terminals of class G3, require higher transfer rates, such as 14.4 kbit/s. One of the growing areas of mobile data transfer that requires higher transfer rates than 9.6 kbit/s is the mobile video service. Services of this kind include e.g. security control by cameras, and video databases. The minimum data transfer rate in video transfer may be, for instance, 16 or 32 kbit/s.
The data transfer rates of the present mobile communication networks are not, however, adequate to satisfy these new needs.
One way to solve the problem is to use multiple TDMA time-slots for communication with a mobile station. In this way, one mobile station may transmit and receive data at higher data transfer rates by multiplexing the data to multiple time-slots (traffic channels). This is termed as multi-slot access technique.
When multiple time-slots are used for communication with one mobile station, problems arise if a sufficient number of traffic channels in not available. This may occur in the call set-up phase or in a handover. Handover refers to transferring the mobile station from one channel onto another channel of the same cell or an adjacent cell during an ongoing call. A problem arises if the mobile station is operating at a high data transfer rate, and the new cell can not provide after the handover a data transfer rate as high as that of the previous cell. One solution would be to interrupt the connection if the quality i.e. the data transfer rate of the data transfer service is not adequate in the call set-up or handover, or after-the handover. However, this solution is not acceptable.